Sensing the presence or absence of material



4? m U U V cmmwu HUULIQ Nov. 30, 1965 R. ROD 3,220,258

SENSING THE PRESENCE OR ABSENCE OF MATERIAL M YUM" I Original Filed May18, 1956 I 9a FIG.3 96 as 9a I I a IMPEDANCE INNCATOR CHANGE on aourROLI SENSOR mamas K76 mmcm'an OR CONTROL MEANS INDICATOR INVENTOR. ROBERTL. ROD

A T TOR KEYS United States Patent M 3,220,258 SENSING THE PRESENCE ORABSENCE OF MATERIAL Robert Louis Rod, Pacific Palisades, Califi,assignor to Aconstica Associates, llnc., Glenwood Landing, Long Island,N .Y., a corporation of New York Original application May 18, 1956, Ser.No. 585,889, new Patent No. 2,990,543, dated June 27, 1961. Divided andthis application Feb. 8, 1961, Ser. No. 87,963 19 Claims. (Cl. 73-290)This invention relates to the sensing of the presence or absence ofmaterial, especially material resting against a wall, and moreparticularly to the sensing of the level of stored material in acontainer, and is a division of my copending application Serial No.585,889, now Patent No. 2,990,543 filed on May 18, 1956. The storedmaterial most commonly is a liquid, but may be a solid, particularlywhen made fluid as when in powdered or granular form.

The primary object of the invention is to provide a method and apparatusfor the above-stated purpose. A more specific object in one form of theinvention is to operate wholly outside the container, without requiringany connection or device inside the container. This is done by settingup a vibration of the wall by means of an electroacoustic transducer,and sensing the difference in impedance or/and frequency of thetransducer assembly resulting from the presence or absence of materialagainst the wall.

A further object is to improve the accuracy of the response obtained.Still another object is to establish an optimum relation between thefrequency of vibration and the thickness and material of the wall. Otherobjects are to apply the aforesaid system to measurement of level, tomaintenance of level, and to the detection of sloshing or tilting.

To accomplish the foregoing general objects, and other more specificobjects which will hereinafter appear, my invention resides in theapparatus elements, and their relation one to another, as arehereinafter more particularly described in the following specification.The specification is accompanied by drawings, in which:

FIG. 1 schematically represents one form of apparatus embodying featuresof my invention;

FIG. 2 discloses a modification;

FIG. 3 schematically represents still another modification;

FIG. 4 is a section through a housing, taken approximately in the planeof the line 44 of FIG. 5;

FIG. 5 is another section through the housing, taken approximately inthe plane of the line 5-5 of FIG. 4;

FIG. 6 is explanatory of apparatus used for level measurement;

FIG. 7 is explanatory of apparatus used for level maintenance;

FIG. 8 is explanatory of apparatus used to detect sloshing or tilting;

FIG. 9 shows how the invention may be used to measure liquid level in anopen reservoir, lake, or river, etc.;

FIG. 10 is explanatory of a modified way to use an electroacoustictransducer; and

FIG. 11 shows the application of the same to an open body of material.

In one form the system may be termed an ultrasonic knucklerapper, whichraps on the wall of a tank to determine the presence or absence of aliquid or solid on the other side. The device operates on acousticprinciples as follows:

An electroacoustic transducer, preferably, but not limited to, apiezoelectric device in the form of a small bar with thin rectangularcross-section, is attached with one 3,220,258 Patented Nov. 30, 1965surface cemented or otherwise secured to the outer surface of the tankor container wall. A small rectangular area of the tank wall surroundingthe crystal preferably is outlined by a relatively heavy rectangularreinforcing frame, so that the vibrations of the wall set up by thetransducer are confined to this area. The area of the wall so outlinedvibrates like a drumhead in synchronism with the electroacoustictransducer. Vibrations are produced by coupling the electroacoustictransducer to a suitable source of high frequency alternating current.

If such a vibrating sensing element is located above the level of liquidor solid material in the tank, it is a high-Q (low damping) resonantsystem. The only possible losses of power in vibrating are very small.They are dynamic losses in the electroacoustic transducer and in thetank wall section which is vibrating as a drumhead, and theinsignificant acoustic radiation losses to the air or vapor above thesurface within the tank. However, if the material covers the position ofthe sensing element on the tank wall, the acoustic radiation losses arevery much larger. This results from the marked difference in the abilityof a mechanical vibrator to radiate power into a liquid (or solid) andinto a vapor. Therefore, when the sensing element is covered by a liquidor solid material, the system becomes a low-Q resonant system.

The detection of this change in the Q-factor can be accomplished byseveral means, the most convenient of which is to use the transducer asthe resonant element in a standard crystal oscillator. The transducer isconnected across the grid circuit of an oscillator tube (or equivalenttransistor emitter), and oscillations take place at a frequencydetermined by the resonant frequency of the system. However, the anodecurrent (both AC. and DC.) is very sensitive to the losses in thetransducer, which appear in the equivalent electrical circuit as adamping resistor in the grid circuit. Therefore, the change betweenconditions with and without material covering the drumhead area causes amarked change in the anode current, which can be detected by any of anumber of different conventional means, some of which are next referredto.

Referring to FIG. 1 of the drawing, the apparatus comprising anelectroacoustic transducer 12 mounted on the wall 14 of a tank 16containing liquid 18. A part of the wall is framed, as indicated at 20.The transducer is vibrated by an oscillator which, in the present case,comprises a vacuum tube 22, tank circuit 24, B battery 26, and an inputcircuit including a coupling resistor 28, a coupling capacitor and abias resistor 32. The apparatus further comprises means to sense thedifference in impedance or/and frequency of the transducer assemblyresulting from the presence or absence of liquid 18 against the framedwall portion 14. By transducer assembly, I mean the transducer 12 withits connected wall area 14. In the present case the anode current flowsthrough a sensitive relay 34 having a movable contact 36 controlling anindicator or/and control means 38, which may be located remotely. Onemay think of the transducer as an impedance which becomes low andheavily damps the oscillator circuit, or one may think of the transduceras changing natural frequency and so differing from the resonantfrequency of the tank circuit 24. In any case there results an abruptchange in anode current, and relay 34 is responsive to that change.

Another means for sensing the presence or absence of a liquid or solidin the drumhead region is to measure the shift in oscillator frequencywhich occurs if the various parameters in the resonant system areproperly selected.

This is illustrated in FIG. 2 of the drawing, in which transducer 40 issecured to the framed portion 42 of the wall of a tank 44 containingliquid 46. The wall portion 42 is framed at 48. As before, the surfacesof transducer 40 other than that secured to wall 42 are dynamicallyfree, but preferably they are secured against static and low frequencyinertial loads by the use of pressure-release supporting materials suchas rubber, neoprene or corprene, in the manner commonly used inelectroacoustic transducer design. This is indicated at 41.

The transducer forms a part of the input circuit of an oscillatorsimilar to that previously described, in comprising a tube 52, a tankcircuit 54, B battery 56, a coupling resistor 58, a coupling capacitor60, and a grid bias resis tor 62. The tank circuit is coupled to afrequency discriminator circuit which may be conventional, and which inthe present case, comprises diodes 64 and 66, symmetrically coupled totank circuit 54 by means of a divided coupling coil or transformersecondary 68, which may be tuned by a capacitor 70. The output is takenacross resistors 72 and 74, shunted by capacitors 76 and 78. There is anadditional coupling capacitor 80 between tank circuit 54 and thediscriminator circuit.

Because of the phase relationships between the primary and each half ofthe secondary 68, each half of the secondary being connected in serieswith the primary through the capacitor 80, the voltages applied to thediodes 64 and 66 becomes unequal as the signal or input swings away fromthe resonant frequency in either direction. As the swing occurs, thevoltages developed across the diode load resistors 72 and 74 connectedin series similarly change. The output voltage depends on the differencein amplitude of the voltages developed across resistors 72 and 74. Thesevoltages are equal and of opposite sign when the oscillator frequency isunchanged, and the output therefore may be balanced to zero, for eitherthe absence or the presence of liquid at the wall portion 42. Then inthe opposite situation there is a distinct unbalance, producing anoutput which may be used to operate an indicator or control means 82.

Another sensing method is to measure the difference in the coupledelectromechanical impedance of the transducer. This is schematicallyillustrated in FIG. 3 of the drawing, in which the electroacoustictransducer 84 is secured to a portion 86 of a tank wall 88 carrying aliquid 90. The wall portion 86 may be framed, as indicated at 92. Inthis case the electrodes of the transducer are on opposite freesurfaces, instead of using the tank wall as one of the electrodes. Thetransducer again may be embedded in pressure-release supporting material85.

The transducer 84 is connected to an impedance change sensor 94, andthis in turn is connected to a suitable indicator or control means 96,which may be located remotely from the transducer. The impedance changesensor housed in rectangle 94 may be a conventional impedance bridge, orany other known form of impedance measuring or impedance responsivedevice.

While a piezoelectric transducer has been mentioned as the preferredelectroacoustic transducer, ferroceramic, magnetostriction,electrostatic, and electrodynamic transducers may also be usedalternatively.

For operating convenience it is preferable to select a vibrationfrequency which is suitably related to the tank wall thickness and thevelocity of sound through the tank wall. For optimum results, the wallthickness at the operating or resonance frequency of the system shouldbe one half-wavelength in thickness, or a multiple of onehalf-wavelength. The loading effects on the transducer then are mostpronounced.

By suitably designing or modifying the circuitry or relays, the responseof the system to a rise in level to the drumhead area can be made eitherrapid or slow, as need be, the slow response being useful when it isdesired to average out sloshing conditions.

From a mechanical standpoint, the device can be made entirelyself-contained within a small box or case affixed to the outside of thetank wall. A flange along the base perimeter of the case may be used asthe stiffener frame previously referred to as confining the vibrationsto a relatively small area in order to improve the definition oraccuracy of the system.

This is illustrated in FIGS. 4 and 5 of the drawing, showing how tankwall may have a housing 102 secured thereto. This is an enclosedhousing, one wall 104 of which is horizontally slotted at 106. Theslotted wall 104 acts as a means to frame the area defined within theslot 106, and to this exposed or windowed area the transducer 108 issecured. The housing 102 may contain the circuitry, and Where exceedingcompactness or miniaturization is required, a transistor may be used inlieu of a vacuum tube.

Some methods of using the invention may be described with reference toFIGS. 6 through 9 of the drawing. In FIG. 6 a series of units are spacedone above the other on a tank wall 110, as is indicated at 112, 114, 116and 118. These units may be like that described in connection with FIGS.4 and 5 each including a transducer and oscillator. Conductors (notshown) lead from the units to a suitable indicator system, which may belocated remotely, and by examining the indicators one can determine theheight of liquid 120 in tank 110, in a series of steps defined by thespacing of the units.

Referring now to FIG. 7, in this case a tank 122 is to be kept filledwith liquid 124 at a predetermined level. A pair of units 126 and 128are disposed above and below the desired level, and are connected bysuitable conductors to a supply valve 130 controlled by a suitable valvecontrol means 132. The arrangement is such that when the liquid level isbelow unit 128 liquid is admitted until the level reaches the unit 126,whereupon the supply is stopped.

A single unit may be used at the desired level, this serving to shut 01fthe supply when liquid is present, and to resume supply when liquid isnot present. If a substanial change in level can be tolerated, thearrangement shown in FIG. 7 has the advantage that the valve willoperate at longer intervals.

In FIG. 8 the tank wall 134 is provided with a plurality of units 136,138 and 140 disposed in horizontal relation. Such units may 'be used todetect sloshing or agitation of liquid in a tank, this being indicatedby the broken line 142. It will be evident that some units will haveliquid present and others not. In the present case there is liquidpresent at units 136 and 140, and not at unit 138. A moment later thesituation may be reversed. Thus, the sloshing may be shown at anysuitable indicator or alarm device, which may be remote from the tank.

A similar array of horizontally disposed units may be used to detect achange in horizontality of a tank. Thus, if the liquid surface isdisposed at an angle, as is indicated by the dot and dash line 144,there will be liquid present at some units and not at others, in aquiescent or steady state, and this may be used as an indication thatthe tank has been tilted, and may be differentiated from the active wavecondition shown at 142.

So far as it is described the measurement has referred to liquidconfined in a tank, and the wall has been the tank wall. However, thisis not essential. For example, referring to FIG. 9 I show an open bodyof water such as a lake, reservoir or river. A plate 152 is immersed inthe liquid, and carries a series of units 154, 156 and 158. The loadingof liquid against the plate 152 will produce a change in impedanceor/and frequency, as previously described, except that here there isliquid on both sides of the plate 152.

It is not essential, even when dealing with material in a tank, that thetransducer be disposed outside the tank, although the ability to workwholly outside the tank is one of the important advantages of thepresent improvement. As an obvious example, the plate 152 in FIG. 9 maybe put inside the tank. Referring to FIG. 10, I there show a transducer160 mounted directly on the inside of a wall 162 of a tank containingliquid 164. Conductors 166 lead to electrodes on the transducer, and thesurface of the device may be coated with a film of suitable insulatingmaterial to protect it against the liquid 164. When the transducer issurrounded by liquid its vibration is damped.

In this particular form of the invention the response may be maderelatively independent of vibration of the tank wall. For that purpose atransducer which vibrates by bending (that is, in contilever) ispreferable. A commercial example is the Bimorph made by the BrushDevelopment Corporation of Cleveland, Ohio. It is made of two slabscemented together and having dissimilar expansion characteristic, thuscausing bending. The bending vibration is damped by the surroundingmaterial 164 when the level rises. In this case wall 162 may be quitethick and rigid.

Referring now to FIG. 11, the height of liquid 170 in an outdoorreservoir, lake, or river, etc. 172, may be measured by a series oftransducers 174, 176, 178 mounted for bending or cantilever vibration.They may be mounted on a rigid post 180, and no vibratile plate or wallis required. Similarly, a post may be used in the tank of FIG. 10, forexample, at the center of the tank.

When vibrating a tank wall, the accuracy is lower with a thicker wall.Also a lower frequency should be used with a thicker wall and a higherfrequency with a thinner wall. However, it is not well to definefrequency solely with reference to thickness, because another factor isthe material of which the wall is made. It is for this reason that Iprefer to speak in terms of wavelength, and the wall thickness is to bea half-wavelength (or a multiple of one half-wavelength) for thevelocity of propagation may vary with the material. A relation based onwavelength inherently allows for differences of material.

When the liquid or solid substance reaches the transducer it loads it,and changes the natural frequency of the assembly, as already described.The frequency is lower when loaded and higher when not, provided thatthe wall thickness is a half-wavelength (or multiple thereof).

In one example of the invention the wall thickness is less than afiftieth of an inch, and the frequency is in the order of megacycles. Byusing a transistor instead of a vacuum tube, the entire circuit shown inFIG. 1, including the battery cell, may be housed within a cube as oneinch on edge. The window for the crystal may be A" high (for accuracy)and 1" wide, and to stiffen the tank wall around the window, the face ofthe cube may itself be used, it being flanged downward from above andupward from below, as already described. The crystal may be disposededgewise to best fit the horizontally elongated window.

It is believed that the construction, theory, and operation of myimprovement, as Well as the advantages thereof, will be apparent fromthe foregoing detailed description. It will be understood that while Ihave referred often to liquid level, the material may be solid insteadof liquid, particularly when in granular form so that it is fluid.

It will be understood that the end electrode connection of FIGS. 1 and 2may be used in FIGS. 3 and 5 in lieu of the side electrode connectionthere shown, and that, conversely, the side electrode connection of FIG.3 and 5 may be used in lieu of the end connection shown in FIGS. 1 and2. The pressure-release material shown in FIGS. 2 and 3 may be used inFIGS. 1, 4, and 5, although not there shown, and, conversely, thepressurerelease material shown in FIGS. 2 and 3 may be omitted ifdesired. The housing shown in FIGS. 4 and 5 which acts also as a framemay be used in FIGS. 1, 2 and 3 in lieu of the separate frame thereshown. The immersed diaphragm arrangement of FIG. 9 may be used in astorage tank, and similarly the immersed post support of FIG. 11 may beused in a storage tank, these forms of the invention not being limitedto outdoor or natural bodies of liquid.

It therefore will be apparent that while I have shown and described myinvention in several preferred forms, changes may be made in thestructures shown, without departing from the scope of the invention, assought to be defined in the following claims. In the claims the termelectroacoustic transducer is intended to be generic to devices whichchange electrical to mechanical energy, and vice versa, at sonic andultrasonic vibration frequencies. It is intended to includepiezoelectric, ferroceramic, magnetostriction, electrostatic, andelectrodynamic transducers. In the claims the expression free to vibrateis not meant to exclude the use of pressurerclease supporting materials,mentioned above. The term fluid means flowable and includes powdered andgranular material as well as liquids.

What is claimed is:

1. Apparatus for sensing the level of a body of fluid material withrespect to a container thereof, which comprises an electromechanicaltransducer mounted at a predetermined level relative to said container,said transducer having a first natural frequency of mechanicalselfresonance, for a given mode of mechanical vibration and beingprovided with a vibrations path in solid material which is an integralnumber of half waves long at said frequency, a substantiallyuninterrupted gaseous path for vibrations propagating between saidtransducer and the surface of said fluid when said fluid is below saidpredetermined level, said gaseous path reducing to zero when said fluidis present at said level, an electronic oscillator circuit includingsaid transducer as the frequency determining element thereof, electricalconductors extending between said oscillator circuit and said transducerfor coupling said transducer into said circuit and setting saidtransducer into mechanical vibration at said natural frequency, saidtransducer having a first value of its Q- factor when it is in contactwith said fluid material and its mechanical vibration is damped therebyand a second value of its Q-factor higher than said first value when itis out of contact with said fluid material, and operator means in saidoscillator circuit having one of two mutually exclusive states dependingon which of said Q-factor values is presented to said oscillator circuitvia said conductors whereby the loading condition of said transducer assensed by said oscillator circuit via said conductors determines thestates of said operator means.

2. Apparatus according to claim 1 in which said transducer is mounted ona Wall of said container, said mechanical vibrations being propagatedthrough said wall, the thickness of said wall being an integral numberof half waves of said vibrations at said frequency.

3. Apparatus for sensing the presence of a fluid material against awall, which comprises an electromechanical transducer mounted on saidwall, circuitry including electrical conductors leading to saidtransducer for setting said transducer into vibration at a desiredfrequency, and additional means connected to said same conductors andcircuitry to sense the difference in an operating characteristic of thesaid transducer assembly resulting from the presence of said fluidmaterial against the wall as compared with said characteristic when saidmaterial is not present against said wall, the said frequency being sorelated to the thickness of the wall that the latter is one or more halfwavelengths of vibrations therein at said frequency.

4. Apparatus for sensing the level of a body of fluid material withrespect to a container thereof, which comprises an electromechanicaltransducer mounted at a predetermined level in a position to be immersedin the body of fluid when the fluid reaches said level, said transducerhaving a first natural frequency of self-resonance for a given mode ofmechanical vibration and being provided with a vibrations path in solidmaterial which is an integral number of half waves long at saidfrequency, a substantially uninterrupted gaseous path for vibrationspropagating between said transducer and the surface of said fluid whensaid fluid is below said predetermined level, said gaseous path reducingto zero when said fluid is present at said level, an electronicoscillator circuit including said transducer as the frequencydetermining element thereof, electrical conductors extending betweensaid oscillator circuit and said transducer for coupling said transducerinto said circuit and setting said transducer into mechanical vibrationat said natural frequency, said transducer having a first value of itsQ-factor when it is in contact with said fluid material and itsmechanical vibration is damped thereby and a second value of itsQ-factor higher than said first value when it is out of contact withsaid fluid material, and operator means in said oscillator circuithaving one of two mutually exclusive states depending on which of saidQ-factors values is presented to said oscillator circuit via saidconductors, whereby the loading condition of said transducer as sensedby said oscillator circuit via said conductors determines the state ofsaid operator means.

5. Apparatus for sensing the level of stored liquid material in acontainer, comprising an electromechanical transducer having a firstnatural frequency of mechanical self-resonance when immersed in agaseous environment, means disposing said body at a predetermined levelin said container, there being a substantially uninterrupted gaseouspath for vibrations propagating between said transducer and the surfaceof said liquid when said liquid is below said predetermined level, saidgaseous path reducing to zero when said liquid is present at said level,electrical conductors leading to said transducer and coupled thereto forinterchanging electrical and elastic wave energy therewith, anelectronic oscillator circuit, said conductors connecting saidtransducer into said circuit as a frequency determining elementtherefor, said circuit being adapted to oscillate at a frequency relatedto said first natural frequency and thereby to cause mechanicalvibration of said transducer at said first natural frequency whenimmersed in a gaseous environment, and an operator in said oscillatorcircuit adapted to assume a first of two mutually exclusive states whensaid circuit is in a first condition and the second of said states whensaid circuit is in a second condition, said circuit being adapted to bein said first condition when said transducer is in contact with and itsmechanical vibration is damped by said fluid, said circuit being adaptedto be in said second condition when said transducer is surrounded by agaseous material having a substantially lower damping effect onmechanical vibration thereof.

6. Apparatus for sensing the level of stored liquid material in acontainer, which comprises a piezoelectric transducer mounted at apredetermined level relative to said container, there being asubstantially uninterrupted gaseous path for vibrations propagatingbetween said transducer and the surface of said liquid when said liquidis below said predetermined level, said gaseous path reducing to zerowhen said liquid is present at said level, said transducer having afirst natural frequency of selfresonance for a given mode of mechanicalvibration, an electronic oscillator circuit including said transducer asthe frequency determining element thereof, electrical conductorsextending between said oscillator circuit and electrodes on saidtransducer for coupling said transducer into said circuit and settingsaid transducer into mechanical vibration at said natural frequency,said transducer having a first value of its Q-factor when it is incontact with said liquid material and its mechanical vibration is dampedthereby and a second value of its Q-factor higher than said first valuewhen it is out of contact with said liquid material, and operator meansin said oscillator circuit having one of two mutually exclusive statesdepending on which of said Q-factor values is presented to saidoscillator circuit via said conductors, whereby the loading condition ofsaid transducer as sensed by said oscillator circuit via said conductorsdetermines the state of said operator means.

7. Apparatus for sensing the level of a body of fluid material withrespect to a container thereof, which comprises an electromechanicaltransducer mounted at a predetermined level relative to said container,there being a substantially uninterrupted gaseous path for vibrationspropagating between said transducer and the surface of said fluid whensaid fluid is below said predetermined level, said gaseous path reducingto zero when said fluid is present at said level, said transducer havinga first natural frequency of mechanical self-resonance, for a given modeof mechanical vibration and being provided with a vibrations path insolid material which is an integral num ber of half waves long at saidfrequency, an electronic oscillator circuit including said transducer asthe frequency determining element thereof, said circuit having a portiontuned to electrical resonance at said frequency, electrical conductorsextending between said oscillator circuit and said transducer forcoupling said transducer into said circuit and setting said transducerinto mechanical vibration at said natural frequency, said transducerhaving a first value of its Q-factor when it is in contact with saidfluid material and its mechanical vibration is damped thereby and asecond value of its Q-factor higher than said first value when it is outof contact with said fluid material, and operator means in saidoscillator circuit in series with said tuned portion and having one oftwo mutually exclusive states depending on which of said Q-factor valuesis presented to said oscillator circuit via said conductors, whereby theloading condition of said transducer as sensed by said oscillatorcircuit via said conductors determines the state of said operator means.

8. Apparatus for sensing the level of a body of fluid material withrespect to a container thereof, which comprises a plurality ofelectromechanical transducer means each mounted at the samepredetermined level relative to said container but in a separate anddistinct location at said level, means coupled to each transducer tosense the difference in an operating characteristic thereof resultingfrom the presence of said fluid material in acoustical contact therewithas compared with said characteristic when said fluid material is not sopresent, and means including said transducer for detecting sloshing ofsaid fluid material in said container.

9. Apparatus for detecting the presence or absence of a body of liquidat a location between upper and lower limits of level and for indicatingthe level of the body of liquid within said limits, comprising a solidbody comprising a conductor of ultrasonic energy formed with a faceadapted to extend between said upper and said lower limit of level todefine the said location and to enter into liquid contactingrelationship with the body of liquid when present at the said location,signal transmitter means mounted on the body and including a transduceradapted to be energized by means including an electric oscillator topropagate ultrasonic energy into the body towards said surface betweensaid limits of level, signal receiving means adapted to be influenced byultrasonic energy reflected back from said surface through said solidbody to said signal receiving means and adapted to assume a condition ofmagnitude proportional to the intensity of said ultrasonic energyinfluencing the signal receiving means, and means adapted to effect asignal in dependence on the magnitude of the condition of the signalreceiving means.

10. An apparatus as claimed in claim 9, wherein the said transducerforms part of the signal receiving means.

11. Apparatus as claimed in claim 10, wherein the transducer includes abody having piezo-electric qualities and a specific inductive capacitywhich varies with intensity of ultrasonic energy reflected back to thetransducer and the change in specific inductive capacity as the level ofthe body of liquid varies between said upper and lower limits is adaptedby varying the value of capacity in the oscillator circuit to determinethe amplitude of oscillation therein.

12. Apparatus for detecting the presence or absence of a body of liquidin relation to a solid body at a location contiguous with a surface ofthe solid body extending between upper and lower limits of level and forindicating the level of the body of liquid within said limits,comprising signal transmitting means including piezo-electric transducermounted on the solid body and having a specific inductive capacityvariable by impressing ultrasonic vibration thereon, an electricoscillator, the transducer being arranged to be energized by saidelectric oscillator for the propagation of ultrasonic energy into thesolid body towards said surface between said upper and lower limits oflevel, the electric oscillator having a tuned circuit with a capacitanceincluding said transducer adapted to control amplitude of oscillation ofthe circuit, signal receiving means connected with said circuit,responsive to electrical condition of the transducer and adapted toassume one condition with the body of liquid at the said location andconsequent small reflection of ultrasonic vibrations from said surfaceto said transducer and another condition with the body of liquid absentfrom said location and consequent large reflection of ultrasonicvibrations from said surface to said transducer and intermediateconditions when the level of the body of liquid is intermediate theupper and lower limits of level and consequent intermediate amounts ofultrasonic vibrations are reflected back from said surface to thetransducer, and means comprising a current sensitive device adapted toeffect a signal in dependence on the amplitude of oscillation in theoscillation in the oscillator circuit.

13. Apparatus for detecting the presence or absence of a body of liquidat a location between upper and lower limits of level and adjacent theinner face of a side wall of a container for the body of liquid and forindicating the level of the body of liquid between said upper and lowerlimits, comprising transmitter means mounted on the outer face of thewall of the container opposite the said location and including atransducer adapted to be energized by means including an electricoscillator to propagate ultrasonic energy into the wall of the containertowards the location and between said limits of level, and signalreceiving means arranged to be influenced by ultrasonic energy reflectedback from the inner face of the wall of the container and adapted toestablish one condition when the level of the body of liquid is abovethe said location, another condition when the body of liquid is absentfrom the location and intermediate conditions when the level of the bodyof liquid is intermediate the said upper and lower limits of level, andmeans including a current sensitive device adapted to effect a signal independence on the magnitude of the condition of the signal receivingmeans.

14. Apparatus as claimed in claim 13, wherein the transducer and theelectric oscillator are adapted to form part of the signal receivingmeans and the transducer is arranged to be influenced by ultrasonicenergy reflected back from the inner face of the wall adapted toestablish one condition in the oscillator circuit when the body ofliquid is absent from the said location, to establish another conditionwhen the level of the body of liquid is above the location, andintermediate conditions when the level of the body of liquid isintermediate the said upper and lower limits of level.

15. Apparatus as claimed in claim 14, wherein means are provided adaptedto control the level of the body of liquid in dependence on themagnitude of the said condition and thereby determine the level of thebody of liquid between said upper and lower limits.

16. Apparatus as claimed in claim 14, wherein the transducer includes abody having piezo-electric qualities and a specific inductive capacitywhich varies with changes in ultrasonic energy reflected back to thetransducer and has different values in dependence on the level of thebody of liquid between said upper and lower limits of level, and changein specific inductive capacity between said values is adapted by varyingthe values of capacity in the oscillator circuit to determine theamplitude of oscillation in the oscillator circuit.

17. Apparatus for detecting the presence or absence of a body of liquidat a location between upper and lower limits of level and for indicatingthe level of the body of liquid within said limits, comprising solidbody means including elastic wave energy conductor means positioned atsaid upper and said lower limit of level for defining said location andfor entering into liquid contacting relationship with the body of liquidwhen present at said location, signal transmitter means coupled to saidsolid body means and including transducer means adapted to be energizedby means including an electric oscillator for propagating elastic waveenergy into conductor means, means for determining damping of saidenergy due to the presence of said liquid at said conductor means andadapted to assume a condition of magnitude proportional to the amount ofsaid damping, and means for efiecting a signal in dependence upon saidmagnitude of said condition.

18. Apparatus for detecting the presence or absence of a body of liquidin relation to solid body means at a location contiguous with a surfaceof said solid body means having portions at least at upper and lowerlimits of level and for indicating the level of the body of liquidwithin said limits, comprising signal transmitting means includingpiezo-electric transducer means mounted on said solid body means, anelectric oscillator, said transducer means being arranged to beenergised by said electric oscillator for the propagation of elasticwave energy into said solid body means, said electric oscillator havinga tuned circuit including said transducer means, means connected withsaid circuit responsive to an electrical condition of said transducermeans and adapted to assume at least one condition with said body ofliquid adjacent said lower limit and another condition with said body ofliquid adjacent said upper limit, and means comprising a currentsensitive device for effecting a signal in dependence on the amplitudeof oscillation in said oscillator circuit.

19. Apparatus for detecting the presence or absence of a body of liquidat a location between upper and lower limits of level, comprisingtransmitter means including transducer means adapted to be energized bymeans including an electric oscillator for propagating elastic waveenergy into said transmitter means at least at said limits of level, andmeans responsive to changes in the amplitude of said energy due toliquid adjacent said transmitter means and adapted to establish at leastone condition when the level of said body of liquid is above said upperlimit and at least another condition when the level of said body ofliquid is below said lower limit, and means for eflecting a signalcorresponding to any of said conditions.

References Cited by the Examiner UNITED STATES PATENTS 2,484,623 10/1949 Heising 73--67.7 2,808,581 10/ 1957 Findlay 340-244 3,019,650 2/1962 Worswick 73-290 FOREIGN PATENTS 582,760 8/ 1933 Germany.

ISAAC LISANN, Primary Examiner. ROBERT L. EVANS, Examiner.

3. APPARATUS FOR SENSING THE PRESENCE OF A FLUID MATERIAL AGAINST AWALL, WHICH COMPRISES AN ELECTROMECHANICAL TRANSDUCER MOUNTED ON SAIDWALL, CIRCUITRY INCLUDING ELECTRICAL CONDUCTORS LEADING TO SAIDTRANSDUCER FOR SETTING SAID TRANSDUCER INTO VIBRATION AT A DESIREDFREQUENCY, AND ADDITIONAL MEANS CONNECTED TO SAID SAME CONDUCTORS ANDCIRCUITRY TO SENSE THE DIFFERENCE IN AN OPERATING CHARACTERISTIC OF THESAID TRANSDUCER ASSEMBLY RESULTING FROM THE PRESENSE OF SAID FLUIDMATERIAL AGAINST THE WALL AS COMPARED WITH SAID CHARACTERISTIC WHEN SAIDMATERIAL IS NOT PRESENT AGAINST SAID WALL, THE SAID FREQUENCY BEING SORELATED TO THE THICKNESS OF THE WALL THAT THE LATTER IS ONE OR MORE HALFWAVELENGTHS OF VIBRATIONS THEREIN AT SAID FREQUENCY.